The semiconductor industry has prospered because of the ability of its technicians to make semiconductor devices ever smaller and often more intricate. They have evolved a constant succession of ingenious ways to process chosen microscopic regions of a larger semiconductor wafer. Advances in this kind of selective area processing are fundamental to the semiconductor arts.
For many years it has been known that the minority carrier lifetime in semiconductor crystals can be reduced significantly by creating displacement damage in the crystal. Since carrier lifetime is an important functional property in many semiconductor devices it has been proposed to use this mechanism for selective treatment of various devices. For example, if the region of a device that is to be active is appropriately masked and the surrounding exposed regions damaged by high energy electrons, ions or equivalent radiation, one can effectively electrically isolate a device, selectively destroy devices in order to program a memory array, create display patterns in electroluminescent bodies, form stripe geometries in heterostructure lasers and do any number of similar selective processes. In some categories of devices, e.g., those often referred to as majority carrier devices and represented most notably by field effect transistors, the important consequence of radiation damage is its effect on the threshold voltage due to changes in the fixed charge and the majority carrier concentration in e.g., a MOSFET, or to the majority carrier concentration along the buried channel of the typical JFET. Both of these electrical characteristics depend on the number of defect states in the semiconductor so that the threshold voltage of selected devices in an array of field effect devices can be modified as desired by selective damage. In all of these procedures the selection depends on a physical mask for the damaging radiation. Mask defects and errors in mask alignment lower device yields, especially with arrays of devices made near the resolution limits of the technology. In the case of a memory array it is possible to re-program the array by annealing the radiation damage and damaging a new set of devices. This requires, obviously, another mask.
The ability to "heal" radiation damage by thermal treatment raises another possibility--that of causing uniform damage in the semiconductor and selectively healing desired regions. This thought is frustrated by the nonselective nature of thermal energy and the time required for the annealing process. At one time it was hoped that the laser would provide a tool for achieving selected area thermal processing. Failures in laser induced diffusion were no doubt matched by similar failures to selectively heal radiation damage. These are accounted for primarily because the heat cannot be confined to the desired area for the length of time required to completely anneal.
According to one aspect of the invention selected areas of a radiation damaged semiconductor are healed by new electronic mechanisms. According to another aspect of the invention selected devices in a device array in which the array has been damaged by radiation are healed by any of several electronic mechanisms. This allows the array to be electrically programmed.
Briefly stated, and to be treated in greater detail below, the three damage healing electronic mechanisms are:1: charge stated enhancement; 2: carrier recombination enhancement; and 3: electric field enhancement.
Healing selected defects can be achieved by the electronic process alone, or the electronic process can be augmented by thermal treatment. The latter is especially useful if the non-selected devices are likely to experience in service electronic conditions that would, but for the absence of thermal augmentation, heal the damage and activate the device.
The damage healing mechanisms are used advantageously to activate selected semiconductor devices in a device array already formed. The devices to be activated can be selected electrically via the electrode contacts to the completed devices. This selection process (or coding process in the case of a memory array) is 100% reliable whereas using the inverse mechanism of selection by damaging certain devices of a "good" array (referred to earlier) involves potential masking failures.